Planetary-gear speed-change transmissions are known, e.g. from ATZ 85 (1983) 6 pages 393 to 398, and from SAE Technical Paper Series No. 820740. Both transmissions consist of a three-speed coupling drive as basic drive, and a fourth speed, which is realized as overdrive, is achieved by means of a group drive arranged in series with the basic drive. These systems have the advantage that tried arrangements are retained in principle, and that the overdrive provided is derived from a simple gear train which is arranged as a group, i.e. a series of gauged drive.
The arrangement of the group drive ahead of the basic drive, as seen in the example of the SAE Technical Paper Series, or after it, as shown according to ATZ, is relevant only in connection with the layout of the drive elements, with both systems having respective advantages and disadvantages.
Four speed transmissions of this type have proven themselves in practice and offer, in conjunction with the arrangement of the fourth speed as overdrive and a torque converter with a bypass clutch, advantages especially with regard to fuel consumption. Also, the shiftability of such a transmission is good, because undesirable simultaneous shifting common with the conventional arrangement of groups in the basic and group drive is avoided by joining the fourth speed to the last speed of the basic drive.
It is a disadvantage that the adaption of the entire drive train is not sufficiently flexible with such a solution drive, and is insufficient with respect to the overall transmission ratio and number of speeds.